The invention is related to the field of ultrasonic non-destructive testing. In particular, the invention relates to an apparatus for measuring "inside" flash found in welded pipes.
Electric resistance welding (ERW) has become the most popular and proven method available for the manufacture of pipe. The ERW process, which includes radio frequency contact, radio frequency induction, low frequency resistance, and D.C. resistance welding, is a forge welding process in which the edges of a metal strip are heated by electric current flow. As shown in FIG. 1, high frequency current enters the tube via sliding contacts and flows along the edges of the metal strip to and from the weld point. The heated edges of the metal strip (or skelp) are forged welded together resulting in protrusions, most commonly referred to as flash, on both the inside and outside of the formed pipe (See FIG. 2).
Outside flash is generally removed by a tungsten carbide cutting tool contoured to the pipe surface. The effectiveness of the outside flash removal process may easily be ascertained and controlled, as outside flash is readily visible.
Inside flash is removed using a trim tool that is positioned at point inside the pipe past the weld point. The trim tool is anchored by a support structure at a point where the edges of the metal strip are still separated. The specifications for inside trim conditions vary with various types and grades of products, but generally call for adherence to tolerances as small as 0.001 of an inch. For some applications complete and clean removal of flash is generally accomplished by undercutting as shown in FIG. 3, such as for drawn over mandrel (DOM) process. Although it is important to have adequate undercut to ensure clean flash removal, excess undercut must be avoided as it cannot be tolerated in successive drawing processes. An excess of remaining flash is undesirable, for example, in line pipe, as the flash can damage pipeline pigs used for both cleaning and the introduction of corrosion control agents. Remaining amounts of flash can also act as crevices that may trap corrosive products. It will be readily understood that the detection and control of inside flash is difficult due to its location within the pipe. In fact, the condition of the inside flash in conventional tube operations is indeterminable until the pipe can be inspected after a downstream cut off operation. As a result, a great deal of scrap pipe can be generated if a condition, such as the misalignment of the trim tool, results in an out of tolerance condition of the inside flash. An apparatus capable of monitoring the condition of inside flash at a point near the welding process could significantly improve the pipe production process by permitting realtime correction of out of tolerance inside flash conditions.
To date, attempts to produce an apparatus capable of non-destructive monitoring of inside flash have met with little success. Specifically, efforts to employ conventional ultrasonic flaw detection systems, such as shear testing systems, have been found unsatisfactory for the detection and monitoring of inside flash. Two major problems have been encountered; the inability of conventional ultrasonic systems to detect with accuracy the presence of inside flash even in static conditions, and the difficulty in locating an ultrasonic system, which employs a water column to provide coupling between an ultrasonic sensor and the pipe, in close proximity to a weld point having a temperature of approximately 1500 degrees fahrenheit. Specifically, with respect to the latter problem, the high temperature of the weld point causes the water to boil thereby creating bubbles that interfere with the ultrasonic signal.
In addition, the structural requirements for such an apparatus are further complicated by the requirement of rapid transverse motion of the ultrasonic sensor across the weld line. The transverse motion is required to generate a weld profile, however, such motion is counter-active to the requirement of accurate positioning of the sensor in order to detect the extremely small inside flash variation.